During the 2 years of its existence, the AHDL has worked to solidify and extend observations that uncovered a previously unappreciated aspect of innate immunity: the requirement that a potent microbial stimulus, lipopolysaccharide (LPS), be inactivated by a host enzyme before homeostasis can be restored following Gram-negative bacterial diseases. The enzyme, acyloxyacyl hydrolase (AOAH), has been the focus of our labs work for many years. We have carried out and/or completed the following studies during 2010-2011: a. Prolonged hepatomegaly in mice that cannot detoxify endotoxin. Small intravenous doses of LPS induce hepatic enlargement in Aoah-/- mice. The liver size increases by up to 80% and remains large for at least 3 weeks. We have explored the basis for this extraordinary phenomenon and found evidence for sinusoidal enlargement, Kupffer cell hyperplasia, and hemophagocytosis. Neutralizing TNF and/or IL-1b could ameliorate the phenotype whereas neutralizing IL-10 resulted in much greater enlargement. Persistently active LPS thus may have profound consequences in the liver. Our results were published this summer in HEPATOLOGY. b. What happens to LPS in vivo? We studied the fate of fluorescent or radiolabeled LPS injected subcutaneously in mice. We found that the LPS drained from the subcutaneous site over many weeks. Most of it passed through the subcapsular sinus and medulla of the draining lymph node(s);very little entered the paracortex or B cell follicles. Approximately 70% of the LPS was inactivated within the subcutaneous site by enzymatic deacylation (AOAH). The LPS gradually accumulated in the liver. We found no evidence that the endotoxin that enters the bloodstream is active (stimulatory). The kinetics of both lymphatic drainage and LPS inactivation had a major impact on the immunological response to the LPS, measured in these experiments as the production of polyclonal IgM and IgG3. The results were recently published in the Journal of Immunology. c. What is the basis for prolonged macrophage tolerance (reprogramming) following exposure to LPS in vivo? Aoah-/- mice, when exposed to small amounts of LPS, develop long-lasting hyporesponsiveness to a second LPS exposure. To understand the molecular basis for this phenomenon, which resembles (and may be a useful model for) the state of immunosuppression that occurs in humans following bacterial infections, we have focused on the peritoneal macrophage. An extensive series of transfer studies (involving the transfer of peritoneal macrophages from mice with one phenotype/genotype into recipient mice of a different phenotype/genotype) have shown us that the trigger for prolonged reprogramming in Aoah-/- macrophages is not, as expected, a fixed change in the cells, but rather the presence of fully active extracellular LPS within the peritoneal cavity. Further studies to test this hypothesis are in progress.